Sustainability Indicators are measurements of groundwater-related characteristics obtained from data collection efforts. Indicators provide evidence about the condition of the groundwater system and (or) how it has changed relative to sustainable groundwater use, which is groundwater use balanced to meet societal, economic, and environmental groundwater needs. Indicators are thus the core measurements used to evaluate groundwater sustainability. To review the USGS study where these indicators were first published as well as other reports see the Upper San Pedro Partnership’s Studies and Reports.
The Upper San Pedro Partnership (USPP) has grouped the sustainability indicators into four main categories: Regional Groundwater Indicators, Near-stream Groundwater Indicators, River Flow Indicators, and Springs Indicators.
Near-stream Groundwater Indicators
The Near-stream Groundwater Indicators group provides evidence about the condition of the ecosystem right along the San Pedro River and enables tracking of how it has changed or is trending over time. Also referred to as “riparian system indicators,” indicators 5 through 7 are focused on the alluvial aquifer adjacent to the San Pedro River, and include: near-stream alluvial-aquifer water levels, near-stream vertical gradients, and annual fluctuation of near-stream alluvial-aquifer water levels.
Indicator 5. Near-stream alluvial-aquifer water levels
Groundwater level below land surface measured in non-pumping wells across the Sierra Vista subwatershed that are screened in the near-stream alluvial aquifer.
Alluvial groundwater (or bank storage) can come from flood flows and enhanced-infiltration projects, as well as from a regional aquifer. Regardless of the source, increases in alluvial-groundwater levels are a direct and immediate indication of an increase in groundwater available to riparian plants and to discharge to the San Pedro River; decreases in alluvial-groundwater levels conversely indicate a loss of available groundwater for plants and the river. The lowest dry-season water levels are well correlated with riparian-system health. Along the San Pedro River, a multi-year declining trend in near-stream alluvial-aquifer levels may imply a decrease in groundwater that is available to the riparian area and as base flow to the stream. An increase in water levels may imply the reverse, an increase in groundwater supply available to the riparian area and the stream.
Indicator 6. Near-stream vertical gradients
The difference in groundwater elevation (relative to sea level) as a function of difference in height of the screens (opening to groundwater) in two different wells that are typically located close to each other.
Near-stream vertical gradient values show whether groundwater is moving toward the stream channel (upward gradient; gaining reach) or from the stream channel toward the deeper regional aquifer (downward gradient; losing reach). The vertical gradient is calculated by taking the water-level elevation in the deep well minus the water-level elevation in the shallow well, and dividing it by the difference in the depth of the screen in the deep well minus the depth of the screen in the shallow well. The greater the magnitude of this value, the stronger the gradient.
Indicator 7. Annual fluctuation of near-stream alluvial-aquifer water levels
The annual range of groundwater depths measured in continuously monitored, near-stream alluvial-aquifer wells.
This indicator is related to the health of the riparian forest—the less water levels fluctuate the healthier the trees. It is best evaluated through a continuous measurement of water levels in near-stream wells, ideally with pressure transducers recording water-level depth at least every 6 hours. The highest annual water level is selected from “winter” months, November to April, and annual dry-season low-water level is selected from summer months, May through September. Typically, the highest annual winter water levels occur in March, but highest winter water levels have been known to occur in all months from November to April. Typically, the lowest summer dry-season water levels occurred in June or July, but in years of less summer rain, such as 2009, the low did not occur until September at some locations.
The Springs Indicators group provides evidence about the condition of available water for springs and enables tracking of how springs are trending over time. This category of indicators is based on characteristics of springs in SPRNCA, and include indicators 13 and 14: springs discharge and springs water quality.
Indicator 13. Springs discharge
The amount of water discharging from a spring source per unit of time measured quarterly at 4 springs and 1 flowing well using a manual method such as a current meter, a flume, or a container of known volume.
The Sierra Vista subwatershed springs include 3 springs on the west side of the river and one on the east side, plus one flowing (also called artesian) well on the east side (the McDowell-Craig Farm well). A spring is a point of groundwater discharge to the surface, and thus spring discharge measurements provide information about trends in groundwater flow from areas of natural or managed recharge. The greater the discharge at a spring, the greater the recharge that must be occurring upgradient of the spring, and vice versa.
Indicator 14. Springs water quality
Surface-water samples from spring flow were analyzed for wastewater and pharmaceutical suites four times spanning 2006 to 2010, collected with passive samplers at Murray, Horsethief, and Lewis Springs.
Murray Spring was sampled 4 times (2006, 2008, 2009, 2010) and Horsethief and Lewis Spring were each sampled once (2009). Water quality analyses included wastewater and pharmaceutical suites as well as field parameters. Passive samplers can detect compounds at lower concentrations than discrete sampling (volumes of water collected at a single location at one point in time). In 2009, passive samplers were deployed at Murray, Horsethief, and Lewis Springs. In 2006 and 2010, only discrete samples were acquired from Murray Springs, and in both 2009 and 2010 discrete samples were also acquired from the Environmental Operations Park (Sierra Vista’s wastewater treatment facility).